Automatic gain expander circuit



F. H. SHEPARD, JR

AUTOMATIC GAIN EXPANDER CIRCUIT Filed July 2, 1938 July l5, 1941 Patented July 15, 1941 AUTMATEC GAIN EXPANDER CERCUI'E Francis H. Shepard, Er., Rutherford, N. J., assigner to Radio Corporation of America, acorporation oi. Delaware Application .nly 2, i938, Serial No. 217,114

(Cl. lfm- 171) Claims.

My present invention relates to amplie-rs provided with automatic gain expansion circuits, and more particularly to a method of, and means for, increasing automatically the sensitivity of an audio amplifier according to a predetermined function.

One of the main objects of my invention is to provide an amplifier having a sensitivity which increases as a function of the square of elapsed time after receiving an initial impulse.

Another important object of this invention is to provide an audio ampliier which is adapted for use in connection with geophysical Work, and whose sensitivity varies directly as a function of `the square of the elapsed time after receiving the initial impulse from a bomb exploded underground; the amplified impulses being recorded in a substantially uniform manner as a result of the sensitivity control.

Another object of the invention is to provide an audio ampliiier with an automatic gain expansion arrangement; the amplifier tubes having positive output electrodes connected in such a manner that no appreciable transients are introduced by the action of vthe volume control.

Still another object of my invention is to provide a plurality of cascaded audio amplifiers provided with special gain control electrodes whose potentials are varied in a positive polarity sense by a grid-controlled gas tube as the amplifier input `amplitude increases; and the .am-pliiier tubes each being provided with plate and screen grid electrodes connected in a common output circuit in such a manner that no appreciable transients are introduced by the action of the gain control arrangement.

Still other objects are yto improve generally the efciency and reliability of high gained audio ampliers for geophysical Work, and more especially to provide such an amplifier in an ecomonical manner.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; lthe invention itself, however, as to both its organiza-tion and meth-od of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically a circuit organization whereby my invention may be carried into eiTe-ct. s

Referring now to the accompanying drawing, the numerals i, i and l denote a'plurality of spaced, sound wave pickeup devices; each of these `devices can be any desired type of microphone. It will be understood that when the present amplier system is employed specifi-cally reflected, or transmitted, when a bomb is eX- ploded near the face of the ear-th in a given locality. By spacing a plurality of microphones over the given locality, and measuring the elapsed time for each microphone in pick-ing up a reiected sound wav-e, it is possible to plot the contour of the strata as well as the probable composition thereoi. Inasmuch as the intensity of the sound, which is reflected from, or transmitted through, the various strata of the earth after the bomb explodes, varies inversely as the square of the distance through which the sound must travel, it is desirable to utilize a sound amplifier whose sensitivity varies directly as a function of the square of the elapsed time after receiving the initial impulse from the exploded bomb. By means of such an amplier the output thereof can be recorded on an oscillograph unit, and it will thus be possible vto compare the various recordings. It will be obvious that in the absence of such sensitivity control weak impulses would be recorded so fe-ebly as to prevent any intelligent comparison of the various recordings.

Considering now the audio amplifier circuit, tube 2 is a tube of the 6L? type whose input grid 3 may be connected to one side of the shunted pick-up devices through a condenser having a magnitude of about 0.1 mi. The opposite side of the pick-up devices is connected to the negative end of the grid bias resistor 5 and the grid side of condenser Il is connected to an adjustable tap E through a. resistor 1, it being understood that adjustment of tap E enables the user to select a proper bias potential for grid 3. The plate 8 of tube 2 is connected to one end of the primary winding 9 of audio transformer i5, whereas the screen grid electrodes il and l2 are connected in common to the opposite end of the primary winding E. The plate 8 and screen grids H and l2 are maintained at a positive potential by connecting ythe mid-point of Winding "9 to the positive terminal of the direct current source i3 through a lead i4. The current source I3 can. for example, apply a positive potential of volts to the plate and screen grids of tube 2.

The negative terminal of current source I3 is connected by lead I5 to the cathode I6 of the amplifier tube 2, yand it will be noted that the heater element I1 of t'he tubes is connected across the resistor 5; the latter is in turn shunted across the 6 volt current source I 8 connected in series with current source I3.

The following audio amplier tube I9 may also be of the 6L? type, and has its input grid connected to the high potential end of the secondary winding 20 of transformer ID. The low potential terminal of winding 20 is connected by lead 2| to the end of resistor 1 adjacent tap 6. The cathode of tube I9 is connected to lead I5, and hence adjustment of tap 6 on bias resistor 5 results in simultaneous control of the bias potentials of the input grids of both .tubes 2 and I9. The audio output transformer 22 has one end of the primary 24 thereof connected to the plate 23 of tube I9, while the opposite end thereof is connected to the screen grids of the tube. Lead i4 is connected to the midpoint of primary winding 24, and therefore 'the plates and screen grids of amplifier tubes 2 and I9 are maintained at a common positive potential. The amplied audio output energy of tube I9 is impressed between the signal grid 25 and cathode 26 of the succeeding audio output tube 21.

The tube 21 may be of the 6L6 type, and the grid 25 thereof is connected to the high alternating potential end of the secondary winding 28 of transformer 22. The cathode 26 of tube 21 is connected to lead I5, while the low potential end of Winding 28 is connected to the negative end of resistor 5; the numeral 29 denotes the lead which connects points of most negative potential of the system and may be considered the ground potential line of the system. The screen grid electrode oftube 21 is connected to lead I4, while the plate of the tube is connected to the lead I4 through the input coil 3l of the oscillograph unit 32. It is not believed necessary to show the recording, or oscillograph, unit 32 in any detail, since those skilled in the art are fully acquainted with the manner of constructing such a recording unit.

It will now be observed that tubes 2, I9 and 21 are arranged as a cascaded audio amplifier with their cathodes connected to a common positive potential point on resistor 5, and with their heater elements connected across current source I8. The input energy of the audio amplifier system is amplified successively by the three tubes, and the amplified output energy is impressed upon the recording unit 32 so that there may be recordedfthe amplified impulses. As stated before, when this system is used for bomb exploration in geophysical work it is necessary to provide an arrangement for increasing the sensitivity of the amplifier as the audio input amplitude decreases. Such automatic increase of sensitivity, or automatic amplier gain expansion, is secured by utilizing a grid-controlled, gas triode 33 which may be of the 885 type. .Such a tube may briey be described, since those skilled in the art are aware of its general characteristics. The function of the grid 34 is to control the starting of plate current. The plate 35 is connected by a switch 36Vto the lead I4 with the result that there will be a 90 volt positive potential applied thereto. The cathode 31 is connected to the ground lead 29 through a load resistor 38, and the grid 34 is maintained at a bias of approximately -12 volts with respect to cathode 31 by connecting grid 34 to the ground lead 29 through a path which includes tapped resistor section 39 and the current source 40.

Audio energy from output tube 21 is applied to grid 34 through a path which includes condenser 4I and the resistor section 42. For a given voltage of grid 34 there is a particular plate voltage at which the discharge will just occur. Once the discharge has been set up it can not be further influenced by grid 34, but it may be stopped by reducing the plate potential below the ionization potential of the gas in tube 33. By opening switch 36 it is possible to stop the discharge. Since the rate of de-ionization is extremely rapid, a drop in plate voltage below the ionization potential causes prompt current cutoff. Hence, it will be seen that a negative voltage on grid 34 either maintains plate current cut-off or promptly loses control, depending on the value of the plate voltage. Direct current voltage de- Velope-d across resistor 38 is applied to amplifier tubes 2 and I9 in a gain-increasing sense. This is specically accomplished by providing electrodes 49 and 4I in tubes 2 and I9 respectively, and connecting them to the cathode end of resistor 38 through lead 42. The latter includes the Variable resistor 43 which functions to adjust the rate of sensitivity increase. The electrodes 4B and 4I act as special gain control electrodes, and are shielded from the signal grids by the positive screen electrodes.

After receiving an initial impulse from the input circuit of the system, the sensitivity increases directly as the square of the elapsed time until a denite sensitivity is reached. Inasmuch as the gain of each amplier stage varies directly as the bias on the controlled grid is reduced, the gain of two stages controlled by the grid bias on grids 43--41 will vary as the square of the change of this bias. If three stages were controlled the gain would vary as the cube of the bias change, etc. Thereafter, the sensitivity of the amplifier remains substantially constant. For audio input amplitudes less than a predetermined amplitude, the grid 34 of the gas tube 33 is biased suiciently negative to prevent discharge of the tube. Consequently, there will be no current flow through resistor 38, and the grids 4B and 4I have minimum potential applied thereto. The grids 4|) and 4I may have a minimum gain bias of about 6.3 v. However, as the audio input amplitude increases above the predetermined amplitude the bias of grid 34 becomes less negative and soon reaches a point where the grid loses control, and the discharge to plate 35 occurs. This means that current ows through resistor 38 thereby increasing the positive potential of electrodes 40 and 4I. The current :dow through resistor 38 increases, as the audio input amplitude decreases in intensity, until a definite sensitivity is reached, and thereafter the amplifier sensitivity remains substantially constant. After the grids 3 reach a zero bias, current is drawn from the cathodes to the grids thereby preventing the latter from going more positive. In order to stop the discharge in tube 33 the switch 33 may be opened; this results in the grid 3B regaining control after switch 36 is closed again. The gas type tube 885 is conveniently used in this circuit because of its negative resistance characteristic; that is, because of the fact that vthe tube is either non-conducting or conducting with a l5 volt drop. Thus, the cathode of the 885 is either at a zero potential or a positive potential l5 volts negative with respect to the +B supply.

The output circuit of each of amplifier tubes 2 and I 9 includes the anode and screen grid electrodes of each tube connected in a balanced manner so as to eliminate transients introduced by the action of the gain control arrangement. The push-pull connection of the plate and screens of each of the tubes 2 and I9 functions to eliminate transients because the changes in current occasioned by the variations in potential of the volume control grid affects the plate and screen currents alike so that like current changes in the two halves of the push-pull transformer can balance each other and result in no change and no voltage output from the secondary of the transformer. There is also provided a path between the gain control connection 42 and the ground lead 29 and which path includes a condenser 50 in series with a switch 5|. To eliminate the transients included by the volume control voltage the switch 5l is opened, a small audio signal is impressed between terminals 52, and tap 6 is adjusted to give minimum audio output. Condenser 50 is for the purpose of timing the rate at which the sensitivity of the amplifier builds up, Switch 5l is for the purpose of introducing a small A. C. signal in the manner described, so that the bias on the grids 3 of the 6L'l tubes can be adjusted until this A. C. signal does not appear in the output of the amplifier. This adjustment insures freedom from effects of transients due to potential changes on the controlled grid for the purpose of volume control. It is pointed out that the transformers Hl and 22 are balanced, standard, high impedance audio transformers. While it is not desired to restrict the present circuit to any specic values, the following circuit constants are given to enable those skilled in the art to construct the present circuit:

Resistor 'l 1 megohm Resistor 5 20,000 ohms Resistor 43 1-10 megohms Resistor 42 1 megohm Resistor 39 0.25 megohm Resistor 38 50,000 ohms Condenser 4 0.1 mf. Condenser 50 1 mf. Condenser 4l 0.1 mf.

While I have indicated and described a system lfor carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited tothe particular organization shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1 In an audio amplier system, at least one ampliiier tube which includes a cathode and an output electrode, a signal input electrode disposed between the cathode and output electrode, at least one auxiliary positive electrode disposed between the cathode and output electrode, an audio output circuit for said tube, a source of audio waves comprising a plurality of sound wave pick-up devices arranged in a predetermined spaced relation, said devices being connected in shunt relation between signal input electrode and cathode, said output electrode and auxiliary electrode being connected in opposition in said output circuit, a gain control electrode disposed in the electron stream between said cathode and output electrode, a grid-controlled gas tube having input electrodes coupled to said output circuit, said gas tube having a characteristic such that it becomes sharply conductive upon the impression upon its input electrodes of audio waves in excess of a desired intensity, and means responsive to the direct current voltage output of said gas tube for rapidly controlling the potential of said gain control electrode in an amplifier gain-increasing sense.

2. In a sound amplifying system of the type used in bomb exploration in geophysical work, the system comprising at least two audio amplifiers arranged in cascade, a plurality of sound wave pick-up devices arranged in shunt relation between the input electrodes of the first of said amplifiers, and said devices being spaced at predetermined distances from each other so as to pick up sound waves reflected from various strata of the earth; the improvement which is characterized by the inclusion of means for increasing the sensitivity of said system as a function of the square of elapsed time after receiving an initial sound iin-pulse, said means comprising a gridcontrolled gas tube coupled to the second amplifier and being responsive to a predetermined low amplitude of audio voltage for producing a direct current voltage, and means for applying said direct current voltage to said amplifiers in a sense sharply to increase the gain thereof.

3. In a wave transmission system, at least one tube provided with signal input electrodes and signal output electrodes, a wave input circuit coupled to said input electrodes, an output circuit coupled to said output electrodes, said tube including a special gain contro-l electrode which is independent of said input and output electrodes, a grid-controlled gas tube having input electrodes coupled to said output circuit, said gas tube having a characteristic such that it becomes sharply conductive upon the impression upon its input electrodes of signal waves of a predetermined amplitude, and means responsive tothe direct current voltage output of said gas tube for rapidly controlling the potential of said gain control electrode.

4. In a wave transmission system as deiined in claim 3, a load impedance disposed in the space current path of said gas tube, and a direct current voltage -connection between a point of positive potential on said impedance and said gain control electrode.

5. In a wave transmission system, a plurality of wave transmission tubes, a source of waves coupled to the input electrodes of the first tube, a wave utilization network coupled to the last of said tubes, a grid-controlled, gas tube responsive to an increase in wave amplitude above a predetermined amplitude level for producing a direct current voltage, means for applying said direct current voltage to said transmission tubes in a sense sharply to increase the gain thereof and means operatively associated with the applying means for timing the rate at which said gain increase occurs.

FRANCIS H. SHEPARD, Je. 

